This invention relates to articles in which different microstructures and properties are preferred for different portions of the article. In particular, this invention provides an article having such differences in structure and properties, together with an apparatus and a process for producing such an article.
There are numerous instances where operating conditions experienced by an article, or a component of a machine, place differing materials property requirements on different portions of the article or component. Examples include a crankshaft in an internal combustion engine, a piston rod in a hydraulic cylinder, planetary gears for an automobile transmission or the metal head of a carpenter""s claw hammer. In a crankshaft, the journals must have hardened surfaces to resist wear during operation, but the crankshaft must also be tough enough to withstand transients in loading. Similarly, a piston rod must have a hard surface to avoid nicks, which might otherwise cause leaks of hydraulic fluid, but toughness to withstand transients in loading is also needed. In these two examples, the requirements may be met by fabricating the parts from nodular iron, or a medium carbon steel, and then induction hardening the articles to obtain the hard surface layer in the desired portions of the articles. The depth of the hardened layer produced by induction hardening is frequently between about 0.03 inch to 0.10 inch. In each of these articles, the surface of the article is differentially austenized, typically within a fraction of a minute, and then quenched to develop a hard martensite surface, which then may be tempered as desired.
A planetary gear for an automobile transmission is typically made from a low carbon steel, masked, then carburized. A carburized surface layer, limited to unmasked portions of the surface and generally less than about 0.04 inch in depth, contains sufficient carbon that it becomes substantially harder than the core of the gear during subsequent heat treatment. The hard carburized layer provides wear resistance in the gear teeth, while retaining toughness in the interior of the gear. Although carburizing is sometimes an alternative to induction or flame hardening, it should be regarded as selective surface alloying, rather than differential heat treatment.
A hammer head must be able to withstand pounding against nail heads, but the claws must have sufficient toughness to withstand extracting nails from wood. In this example, the entire striking end of the steel hammer head is austenized, in a minute or two, and then the head is quenched and tempered. This example differs from the crankshaft in that the entire striking end of the hammer is differentially heat treated, rather than just a thin surface layer.
One common feature of the well-known differential heat treatment processes employed in these examples is that each is applied to iron-carbon alloys, where carbon is the atomic species essential to hardening. Because carbon atoms diffuse so rapidly in iron-carbon alloys, each differential heating process can be performed within a few minutes. There is sufficient latitude in austenizing that it is generally not necessary to accurately control the temperature distribution within the differentially heated portions of the articles. Thus, it is generally not necessary to make any provision in the process for keeping the portions of the articles not being heat treated cool.
A turbine disk for a gas turbine engine is an example of another type of article where different properties in various portions of the article are preferred. Such disks are typically made from nickel-base superalloys, because of the temperatures and stresses involved in the gas turbine cycle. In the hub portion where the operating temperature is somewhat lower, the limiting material properties are often tensile strength and low-cycle fatigue resistance. In the rim portion where the operating temperature is higher because of proximity to the combustion gases, resistance to creep and hold time fatigue crack growth (HTFCG) are often the limiting material properties. HTFCG is the propensity in a material for a crack to grow under cyclic loading conditions where the peak tensile strain is maintained at a constant value for an extended period of time. By contrast, in conventional low-cycle fatigue testing the peak tensile strain is reached only momentarily before reduction in the strain begins.
It has not heretofore been possible to conveniently and reliably heat treat a disk to obtain such a combination of different properties in the different regions of a disk. As a consequence, most turbine disks have been heat treated with a process designed to provide a compromise set of properties throughout the entire disk. The various conditions which, taken together, have created such a formidable problem for heat treating, include the following. The disk itself, particularly for a large aircraft gas turbine engine, is generally about 25 inches in diameter. The rim portion of a disk, which must have the same properties throughout its extent, is an annular ring whose dimension in both axial and radial directions is greater than about 2 inches. These dimensions indicate that a large volume of metal must be heated. The nickel-base superalloys must be heated to temperatures above about 2000xc2x0 F., for times of two hours or longer, to achieve the structure which provides the improved creep and HTFCG resistance needed for this application. The hub portion of the disk, however, must be kept below about 1900xc2x0 F. to avoid altering its structure and properties.
The preceding combination of problems has been so formidable that other approaches to developing turbine disks having different properties in their hub and rim portions have been developed. One such approach, which provides a dual alloy disk by forge enhanced bonding of two different alloys for the rim and hub portions of the disk, is described in U.S. Pat. No. 5,100,050, assigned to the assignee hereof, which is incorporated herein by reference. It is noted that while the present invention was developed to provide differential heat treatment, and the resulting differences in properties between different portions of an article, in an article comprised of a single alloy, it may also be advantageously employed in heat treating a dual alloy disk made by the referenced process, or by any other appropriate process, in which the rim and bore or hub must be heat treated at different temperatures to achieve optimum properties in each.
The present invention fulfills the need for a differentially heat treated article, and an effective apparatus and process for providing such an article, and provides related advantages.
The present invention provides a differentially heat treated article, such as a disk of the type employed in turbine sections of gas turbine engines, together with an apparatus and a process for accomplishing such differential heat treatment. As described herein, the present invention contemplates heating a rim portion of a disk to a substantially uniform temperature which is higher than the hub portion of the same disk, such that the material in the rim portion of the disk be given a different heat treatment, in this case at a higher temperature than the material in the hub portion of the disk. As a consequence of the difference in heat treatment temperatures, the mechanical properties developed in those two portions of the disk will be different.
In one embodiment of the present invention, a turbine disk is made from a nickel-base superalloy that can be hardened by the development of a precipitate of the gamma-prime phase. The disk is heat treated, using conventional technology, to achieve the properties required in the hub portion of the disk. Such requirements generally emphasize high tensile strength and resistance to low-cycle fatigue over creep and HTFCG resistance. The disk is then differentially heat treated to raise the temperature in its rim portion high enough to permit grain growth in the rim portion, while keeping the hub portion at a substantially uniform temperature which is low enough to prevent significant changes in the previously developed properties. The larger grain size thus developed in the rim portion of the disk generally improves the resistance to creep and HTFCG in the rim portion, which is frequently a significant advantage in turbine design.
A disk given such a differential heat treatment becomes a dual property disk. It is contemplated that such a heat treatment is applicable to a monolithic disk, where the entire disk is comprised of the same alloy, or to a dual alloy disk, where the rim and hub regions are comprised of different alloys.
The present invention provides an important advance in the art of differentially heat treated articles, and apparatus and process for manufacturing such articles. Other features and advantages of the present invention will be apparent from the following more detailed description of the invention, which, taken in conjunction with the accompanying Figures and Examples, illustrate, by way of example and not by way of limitation, the principles of the invention.